1. Field of the Invention
The present invention relates to a vibration damping device applicable as an engine mount for use in an automotive vehicle, for example, and in particular to a fluid filled vibration damping device in which damping effects are obtained based on flow action of a non-compressible fluid sealed in the interior.
2. Description of the Related Art
A fluid filled vibration damping is known as one type of damping connectors or damping supports mounted between members forming a vibration transmission system. JP-Y-4-33478 shows one example of the fluid filled vibration damping device. This type of damping device typically includes: a first and a second mounting member elastically connected together by a rubber elastic body; a pressure receiving chamber in which part of the wall is composed of the rubber elastic body; an equilibrium chamber in which part of the wall is composed of a readily deformable flexible film; an non-compressible fluid being sealed in the pressure receiving chamber and equilibrium chamber. Damping effects are brought about based on the resonance action of the fluid flowing through the orifice passage that is formed so as to connect the pressure receiving chamber and equilibrium chamber to each other, as a result of the relative change in pressure between the pressure receiving and equilibrium chambers when vibrations are input between the first and second mounting members.
Damping effects based on the resonance action of the non-compressible fluid flowing through the orifice passage are readily brought about only in a specific pre-tuned range of frequencies. A hydraulic absorption mechanism based on a movable panel has been proposed in order to improve damping performance by avoiding the development of extremely high dynamic spring, particularly during the input of vibrations in frequency ranges higher than the tuned frequency range of the orifice passage. In this hydraulic absorption mechanism, a housing space is generally formed in the partition member dividing the pressure receiving chamber and equilibrium chamber, and a movable panel is micro-displaceably disposed in the housing space. The housing space is formed via through holes in the pressure receiving chamber and equilibrium chamber, so that the pressure in the pressure receiving chamber is exerted on one side of the moveable panel, and the pressure in the equilibrium chamber is exerted on the other side.
The displacement of the movable panel due to differences in pressure between the pressure receiving chamber and the equilibrium chamber allows minute fluctuations in pressure in the pressure receiving chamber to escape into the equilibrium chamber during the input of high frequency vibrations. Because of the substantial vibration amplitude during the input of low frequency vibrations for which the orifice passage has been tuned, the movable panel comes into contact with and overlaps the inner surface of the housing space, effectively closing off the through holes. This prevents the absorption of pressure in the pressure receiving chamber by the hydraulic absorption mechanism, resulting in the effective production of relative pressure fluctuations between the pressure receiving chamber and equilibrium chamber, while also ensuring a sufficient flow of fluid through the orifice passage between the two chambers to bring about damping effects by the orifice passage.
However, during rapid pressure fluctuations in the pressure receiving chamber upon the input of vibrations of greater amplitude in this type of hydraulic absorption mechanism, the movable panel strikes the inner surface of the housing space. The impact of the movable panel striking the surface tends to result in noise and vibration. If used as an automobile engine mount, for example, the noise will sound disagreeable to the operator when the engine is cranked or while driving over bumps, detracting from the driving experience.
To address such problems, JP-Y-4-33478 proposed constructing a movable pane with a rubber elastic panel, and integrally forming a small protrusion in the form of a lip on the surface, so that the protrusion could absorb impact when struck. Although this type of small protrusion is effective against low energy strikes, it is not very effective during rapid and extensive pressure fluctuations in the pressure receiving chamber, and another way of dealing with this problem is needed.